WASHINGTON — President Barack Obama is launching the nation's most sweeping effort to curb gun violence in nearly two decades, urging a reluctant Congress to ban military-style assault weapons and high-capacity ammunition magazines like those used in last month's massacre of 20 elementary school children in Newtown, Conn.

The broad package Obama will announce today is expected to include more than a dozen steps the president can take on his own through executive action. But those measures will be limited in scope, and in some cases, focused simply on enforcing existing laws.

Congress would have to approve the bans on assault weapons and ammunition magazines holding more than 10 bullets, along with a requirement for universal background checks on gun buyers.

Some gun control advocates worry that opposition from Republicans and conservative Democrats, as well as the National Rifle Association, will be too great to overcome.

"We're not going to get an outright ban," Rep. Carolyn McCarthy, D-N.Y., said of limits on assault weapons. Still, McCarthy, a leading voice in Congress in favor of gun control, said she would keep pushing for a ban and hoped Obama would as well.

Obama will announce his proposals in a midday event at the White House, flanked by children who wrote to him about gun violence following the massacre of 20 students and six adults at Sandy Hook Elementary School. Obama has pledged urgent action to prevent future mass shootings, and his plan is swift by Washington standards.

The president's framework is based on recommendations from Vice President Joe Biden, who led a wide-ranging task force on gun violence. Beyond the gun control measures, Biden also gave Obama suggestions for improving mental health care and addressing violent images in video games, movies and television.

The vice president's proposals included 19 steps that could be achieved through executive action.

Obama may order the Justice Department to crack down on people who lie on background checks; only a tiny number are now prosecuted. Such a step has support from the National Rifle Association, which has consistently argued that existing laws must be enforced before new ones are considered.

He also could take steps ordering federal agencies to make more data on gun crimes available and conduct more research on the issue, something Republican congressional majorities have limited through language in budget bills. And he may order tougher penalties against gun trafficking and give schools flexibility to use grant money to improve safety.

Gun control proponent Rep. Bobby Scott, D-Va., who met with Biden on Monday, said the president is also likely to take executive action to ensure better state reporting of mental health and other records that go into the federal background check database.